re: change in orientation of the fringing fields
Yes. That's a very good point.

re: Vss plane being cut back instead
In the original application of the 20H rule, it probably wouldn't have mattered.
In later applications (and alterations) other than the original, it might have mattered.

As as side note:
The original 20H rule came out of a "field" problem being solved. From
that standpoint, it might have actually been true to some extent to have
called it a "screwy SI rule" because it really wasn't an SI problem to begin
with. Knowledgeable folks dealing with SI issues would never have
classified the original use as an SI issue. Subsequent uses of the 20H rule
may indeed have blurred the lines a bit.

Michael E. Vrbanac

"D. C. Sessions" wrote:

> Keeping in mind that There Is No Ground, what happens if
> the Vss plane is the one cut back?
>
> As an alternate consideration, note that the stepback would
> change the orientation of the fringing fields, which would
> interact with the enclosure to potentially limit net radiation.
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com>
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